1. Field of the Invention
This invention relates generally to phonograph record players, and more particularly is directed to an improved inside force cancelling device for the tone arm of a record player.
2. Description of the Prior Art
In general, during the playing of a phonograph record disc, a pickup stylus carried by a tone arm is urged, with a predetermined stylus pressure, against the rotated phonograph record disc so as to engage in, and track the spiral record groove of the disc. Such stylus pressure gives rise to a frictional force acting on the tone arm in a direction tangential to the record groove at the point where the latter is engaged by the stylus. Since the tone arm usually is provided with an angled or offset end portion and over-hang in order to compensate for any tracking error, the described frictional force has a first component extending through the substantially vertical axis about which the tone arm is swingable during tracking of the record groove, and a second component substantially at right angles to the first component and urging the tone arm to swing in the inward direction, so as to press or force the pickup stylus against the inner side wall of the record groove. Such second component of the frictional force is usually referred to as the "inside force" and, if allowed to act on the tone arm, causes the forces of the pickup stylus against the opposite side walls of the record groove to be unequal. More specifically, as a result of the inside force, the force of the pickup stylus against the inner side wall of the groove is increased while the force of the pickup stylus against the outer side wall of the record groove is decreased, and such unequal pressures or forces of the pickup stylus against the opposite side walls of the record groove deleteriously affect the reproduction of the recorded sound. For example, the unequal forces may cause abnormal abrasion of the stylus tip and increased distortion particularly in the case of a stereophonic recording in which the decreased pressure or force of the stylus against the outer side wall of the groove results in the imperfect tracking or tracing of such side wall in which the right-hand channel sounds are recorded. The undesirable effects of the inside force are particularly acute in the case of record players having cartridges intended to operate with a low stylus pressure.
Various devices have been proposed for cancelling or counteracting the inside force acting on the tone arm. For example, in one existing inside force cancelling device for the tone arm of a record player, a bell-crank having a substantially vertical arm and a substantially horizontal arm is pivoted on the base of the tone arm mount and has a weight adjustable along the horizontal arm so as to urge the vertical arm against a rod extending radially from the shaft of the mount which turns with the tone arm in response to tracking of the spiral record groove. The parts are arranged so that the force of the vertical arm of the bell-crank against the radial rod produces a turning moment acting on the tone arm in the direction opposed to the turning moment resulting from the inside force. However, the inside force acting on the tone arm varies with changes in the radial distance from the center of the phonograph record disc to the position of the pickup stylus in the spiral record groove. In many instances, such variation of the inside force is not unidirectional, that is, as the mentioned radial distance is progressively decreased during playing of the phonograph record disc, the inside force may initially decrease to a minimum value and thereafter increase. It will be apparent that the turning moment applied to a tone arm by the above described existing inside force cancelling device does not vary in correspondence with such non-uniform variation of the inside force and, therefore, is incapable of accurately cancelling or compensating for the latter.
In another existing or proposed inside force cancelling device, the previously described device is supplemented by a weight suspended from one end of a string having its other end connected to the rod extending radially from the vertical mounting shaft for the tone arm. Intermediate its ends, the string engages a guide which is located on the mounting base so that, as the radial rod moves in a horizontal plane in response to turning of the tone arm, the end portion of the string secured to the rod forms a varying angle relative to such horizontal plane, and thereby varies the horizontal component of the force applied to the rod, and hence to the tone arm, as a result of the suspended weight. Although the last mentioned device may be adjusted to apply a turning moment to the tone arm that approximately counteracts the inside force, even this relatively complicated and fragile arrangement is incapable of accurately compensating for the inside force at all positions of the pickup stylus in the spiral record groove.
It is further to be noted that the above described existing inside force cancelling devices always act on the tone arm to urge the latter to swing in the outward direction so that, when the stylus pressure is being adjusted, for example, after replacement of a cartridge or the like, the inside force cancelling device may cause inadvertent swinging of the tone arm in the outward direction and thereby damage the stylus tip. Accordingly, during adjustment of the stylus pressure, it may be necessary to remove each of the existing inside force cancelling devices from the associated record player or to manually hold such device for preventing the operation thereof.